The invention relates to an image forming apparatus in which a toner image formed on an image bearing member is transferred to a sheet which is being transported on an endless belt rotatably stretched over a plurality of rollers including a driving roller.
Most of recent image forming apparatuses are multifunctional in that they can operate under multiple processing modes such as copying mode, printing mode, or facsimile mode. Also, such recent image forming apparatuses have high-speed printing capability and achieve a processing rate of one or more sheets per second.
To perform high-speed processing, such apparatuses consecutively feed a plurality of sheets thereinto so that the sheets undergo consecutive image forming processes.
The apparatuses are provided with a sheet feeding sensor for detecting that a sheet starts being fed, and a sheet ejection sensor for detecting that a sheet is ejected. The apparatuses determine where therein a sheet as fed is positioned, based on sheet transport speed and time elapsed since the sheet starts being fed with reference to a sheet feeding start time as detected by the sheet feeding sensor.
However, it is difficult to maintain a constant sheet transport speed because of various types of sheets to be used for image formation and because of various conditions under which the apparatuses may be used. A decrease in sheet transport speed in a part of sheet transport path prevents a sheet from being transported timely to sheet transporting members positioned downstream of the part. At worst, the delay results in a sheet jam. A jammed sheet also may damage essential components such as an image bearing member.
As a solution to the foregoing problems, JP H10-161436A discloses an image forming apparatus in which a charging voltage for recharging a toner image carried on a sheet is adjusted according to a potential of the sheet as measured. Toner on a sheet being transported on a transport belt needs to be re-charged so as to be more attracted to the sheet before the sheet is transported into the fusing device. If the toner is excessively charged, however, the sheet is also attracted to the transport belt and thus may cause a jam. The apparatus of JP H10-161436A is intended for preventing such a sheet jam.
JP H11-202647A proposes another solution. An image forming apparatus as disclosed by JP H11-202647A detects whether a sheet passing beyond a portion of an intermediate transfer belt which faces a second transfer device remains on the intermediate transfer belt. If the sheet remains on the belt, an outer second transfer roller provided in the second transfer device is moved out of contact with a surface of the belt. Then, an image bearing member and the belt are deactivated so as to stop transporting the sheet after a predetermined time elapsed since deactivation of the image bearing member and the belt.
A sheet to which a toner image is transferred from the image bearing member at a transfer position is attracted to a circumferential surface of the image bearing member by a surface potential thereof. Thus, a separating claw is provided at a separation position located downstream of the transfer position on the circumferential surface of the image bearing member, in order to separate the sheet from the circumferential surface. The separating claw is moved selectively in or out of direct contact with the image bearing member, according to sheet transport timing.
If there is a delay in transporting a sheet, however, a leading end of the sheet reaches the separation position with the separating claw away from the image bearing member. Without being separated from the circumferential surface of the image bearing member, the sheet is guided by rotation of the image bearing member to a cleaning section provided downstream of the separating claw, so that the sheet gets jammed in the cleaning section. The sheet jam in the cleaning section may damage the cleaning section itself and the surface of the image bearing member. Further, clearing a sheet jam in the cleaning section is a difficult task which an operator finds rather bothersome.
A feature of the invention is to provide an image forming apparatus in which an image bearing member is deactivated, according to determination that a sheet is not separated from the image bearing member, when a sheet is not detected at a predetermined timing at a predetermined position located downstream of a transfer position along a sheet transport path. The configuration prevents occurrence of a sheet jam in a cleaning section, thereby saving an operator from having to do the difficult task of clearing the sheet jam.